Assaying of biological specimens is routine laboratory procedure, both in clinical as well as in the research laboratory. Examples of specimens which are routinely tested consist of urine, blood and others. A critical blood parameter which needs to be determined, primarily in hospital emergency rooms and intensive care units, as well as in patients prior to operation, concerns platelet function. U.S. Pat. No. 5,523,238 describes a cone and plate device and method for determining platelet function in a primary hemostasis. In accordance with this method, a blood sample or a platelet containing fraction thereof is introduced into a well-like vessel (plate member) with a flat bottom on which plasma proteins are immobilized. A cone device is then rotated in the vessel and as a result of shear forces which develop on the surface blood platelets adhere and aggregate on the plasma protein coated surface of the plate member. The platelet function can then be determined based on the pattern of adhered aggregated blood platelets.